A Study On Intergranular Corrosion Resistance And Grain Boundary Character Distributions Of Cold Rolled And Annealed 316L Stainless Steels

316L austenitic stainless steels with extreme low carbon have been widely utilized in chemical,petrochemical,fertilizer and atomic energy industries as structure materials due to their excellent mechanical properties and corrosion resistance. However,the 316L stainless steels often failed in aggressive conditions with high temperature,pressure and erosive environments due to the serious intergranular corrosion.Therefore,in order to extend the service lifetime and maintain system stability,the intergranular corrosion resistance of 316L stainless steels should be further improved.It is well known that the configuration and geometrical characteristics of grain boundaries have great impact on the mechanical,physical and chemical properties of polycrystalline materials such as intergranular corrosion,so it is possible to design and control grain boundary via proper thermomechanical treatments by grain boundary engineering(GBE),in order to improve the intergranular corrosion resistance of 316L stainless steels.In the structural materials reviewed,the very high frequencies of CSL boundaries were shown in grain boundary engineered materials(GBEMs)with 304 and 316 austenitic stainless steels.The very high CSL frequencies resulted in optimum grain boundary character distributions(GBCDs),which were described as uniform distributions of dense CSL boundaries and consequent discontinuity of the random boundaries in the grain boundary network in the 304 and 316 GBEMs.A notable advantage of the GBE process for the 304 GBEM was its one-step slight strain plus annealing process,in contrast to other GBE processes,which required several iterations of strong strain plus annealing for favorable GBCD or improvement of properties.The simpler one-step process would be preferable for industrial applications.However,the study on the intergranular corrosion resistance and grain boundary character distributions of 316L stainless steels processed by one-step slight strain plus annealing have not been reported.Therefore,in this paper we studied the intergranular corrosion and grain boundary character distributions of the commercial 316L stainless steel samples,which were thermomechanically processed by cold rolling for a reduction of 0%,3%,5%and 10%,respectively,and subsequent annealing recrystallization at 967℃for 72 h.The intergranular corrosion tests and electron backscatter diffraction(EBSD)were used to study the intergranular corrosion and grain boundary character distributions of cold rolled and annealed 316L stainless steels samples respectively.The results of intergranular corrosion tests showed that the corrosion degree of the thermomechanically processed samples was lower than the base material's(BM), especially both the corrosion degree and the corrosion rate of the sample 2~# (r3%-a967℃-72h)were lowest in the four samples.A study on the GBCD indicated that the GBCDs of 316L stainless steel samples were improved and the amount of low-∑coincidence site lattice(CSL)boundaries increased by one-step pre-strain plus annealing.The specimen exhibited a maximum∑3,summation of 29 and 227,and all low-∑CSL frequency after 3%pre-straining and subsequent annealing at 967℃for 72 h.It was a remarkable characteristic of twin-induced GBCD optimization that the special low-∑CSL boundaries were mostly (more than 90%)∑3~n boundaries.Moreover,there were some crooked or slightly crooked∑3 boundaries which were in the general high angle boundaries network, except some straight∑3 boundaries in the thermomechanically processed samples.Based on the results of the intergranular corrosion tests and the study on the GBCD,we can see that the low-∑CSL frequency in thermomechanicaUy processed sample 2~# was lower than that in the BM's,but the corrosion rate of the sample 2~# was only about one fifth of the BM's.The GBCD containing high fraction special boundaries which sufficiently interrupt the connectivity of high angle boundary networks in the sample 2~# was the root of the great improvement of the intergranular corrosion resistance of 316L stainless steels.When three grain boundaries met at the same junction during the grain boundaries migration,the interaction of the polymerization-decomposition occurred among the three grain boundaries to form four grain boundary types,which contained:(a)3 low-∑CSL boundaries,or(b)2 low-∑CSL and 1 random boundaries,or(c)1 low-∑CSL and 2 random boundaries and or(d)3 random boundaries,so as to change the GBCDs of 316L stainless steels.